1. Field of the Invention
The present invention relates to a thin film transistor (TFT) having an oxide semiconductor layer as a channel and a display unit including the same.
2. Description of the Related Art
An oxide semiconductor composed of, for example, a zinc oxide, an indium gallium zinc oxide (IGZO) or the like shows superior characteristics as an active layer of a semiconductor device. In recent years, development has been promoted in an effort to apply the oxide semiconductor to a TFT, a light emitting device, a transparent conducting film or the like.
For example, in the TFT including the oxide semiconductor, electron mobility is high and its electric characteristics are superior compared to the existing TFT including amorphous silicon (a-Si: H) used for a liquid crystal display unit as a channel. Further, the TFT including the oxide semiconductor has an advantage that high mobility is able to be expected even at low temperature around room temperature.
Meanwhile, it has been known that in the oxide semiconductor, the heat resistance is not sufficient, and thus due to heat treatment in a manufacturing process of the TFT, oxygen, zinc and the like are detached and lattice defect is formed. The lattice defect results in forming an electrically shallow impurity level, and causes low resistance of the oxide semiconductor layer. This results in normally-on type operation, that is depression type operation in which a drain current is flown without applying a gate voltage, the threshold voltage is decreased as the defect level is increased, and the leakage current is increased.
Thus, in the past, it has been known that oxygen annealing is performed under high temperature after a TFT is formed and thereby oxygen is supplied to an absent portion in which oxygen is lacked or oxygen is detached in the oxide semiconductor layer to restore the characteristics as described in, for example, Japanese Unexamined Patent Application Publication Nos. 2006-15529 and 2006-165532.